LTE-2872U IR Emitter Datasheet - 5mm Package - 1.6V Forward Voltage - 940nm Wavelength - English Technical Documentation

1. Product Overview

The LTE-2872U is a high-performance infrared (IR) emitter diode designed for reliable operation in sensing and detection applications. Its core function is to emit infrared light at a peak wavelength of 940 nanometers, which is invisible to the human eye but ideal for electronic detection systems. The primary application highlighted in the datasheet is for smoke detectors, for which the component holds UL approval, underscoring its reliability and safety for critical life-safety equipment. The device is offered in a low-cost, clear transparent plastic end-looking package, providing a narrow beam pattern that enhances directionality and sensing accuracy.

1.1 Core Advantages and Target Market

The key advantages of the LTE-2872U series stem from its specific design choices. It is mechanically and spectrally matched to companion phototransistors in the LTR-3208 series, ensuring optimal performance in emitter-detector pairs commonly used in slot-type sensors (e.g., for paper detection in printers, object sensing). This matching simplifies design and improves signal integrity. The narrow beam characteristic increases the intensity over a smaller area, improving signal-to-noise ratio in aligned systems. The use of a Gallium Aluminum Arsenide (GaAlAs) window layer on a Gallium Arsenide (GaAs) substrate is a standard technology for efficient IR emission. The primary target market is industrial and consumer electronics requiring robust, low-cost infrared sensing, with a certified niche in smoke detection systems.

2. In-Depth Technical Parameter Analysis

The datasheet provides absolute maximum ratings and detailed electrical/optical characteristics, which are critical for circuit design and reliability assessment.

2.1 Absolute Maximum Ratings

These ratings define the limits beyond which permanent damage may occur. The device can dissipate up to 250 mW of power. The continuous forward current is rated at 150 mA, while a much higher peak forward current of 3 A is allowed under pulsed conditions (300 pps, 10 µs pulse width), which is useful for driving high-intensity short bursts. The maximum reverse voltage is 5 V, indicating the diode's limited tolerance to reverse bias. The operating temperature range is from -40°C to +85°C, and storage can be from -55°C to +100°C, making it suitable for harsh environments. Lead soldering temperature is specified as 260°C for 5 seconds at a distance of 1.6mm from the package body, providing guidance for assembly processes.

2.2 Electrical & Optical Characteristics

Parameters are tested at a standard forward current (I_F) of 20 mA and an ambient temperature (T_A) of 25°C. The forward voltage (V_F) typically ranges from 1.2V to 1.6V. The reverse current (I_R) is a maximum of 100 µA at a reverse voltage (V_R) of 5V. The peak emission wavelength (λ_Peak) is 940 nm, and the spectral bandwidth (Δλ), defined as the half-width, is 50 nm. The viewing angle (2θ_1/2) is 16 degrees, confirming the narrow beam specification.

3. Binning System Explanation

The LTE-2872U employs a rigorous binning system for its radiant output, which is crucial for applications requiring consistent optical performance. Two key parameters are binned: Aperture Radiant Incidence (E_e, in mW/cm²) and Radiant Intensity (I_E, in mW/sr).

3.1 Radiant Output Binning

The datasheet lists multiple bins (A, B, C, D1, D2, D3, D4) for both E_e and I_E. The bins represent sorted ranges of optical power. For example, Bin A for Radiant Intensity has a typical range of 3.31 to 7.22 mW/sr, while Bin D4 starts from 17.17 mW/sr. This allows designers to select a component with the precise output level needed for their application, ensuring adequate signal strength without over-specifying. Higher bin numbers generally correspond to higher efficiency or output devices. Designers must consult the specific bin codes when ordering to guarantee the required performance.

4. Performance Curve Analysis

The datasheet includes several typical characteristic curves that illustrate device behavior under varying conditions.

4.1 Spectral Distribution

Figure 1 shows the spectral distribution, peaking sharply at 940 nm with the aforementioned 50 nm half-width. This curve is vital for ensuring compatibility with the spectral sensitivity of the paired detector (like the LTR-3208).

4.2 Forward Current vs. Forward Voltage

Figure 3 depicts the IV (Current-Voltage) characteristic. It shows the exponential relationship typical of a diode. The curve allows designers to determine the necessary drive voltage for a desired operating current, which is essential for designing the current-limiting circuitry.

4.3 Relative Radiant Intensity vs. Forward Current

Figure 5 shows that the optical output (radiant intensity) is nearly linear with forward current in the typical operating range. This linearity simplifies modulation and control of the light output.

4.4 Relative Radiant Intensity vs. Ambient Temperature

Figure 4 is critical for understanding thermal effects. It shows that the radiant intensity decreases as the ambient temperature increases. This derating must be accounted for in designs intended to operate over the full temperature range, especially near the upper limit (+85°C), to ensure sufficient signal margin.

4.5 Radiation Diagram

Figure 6 provides a polar radiation pattern, visually confirming the 16-degree viewing angle. The pattern shows the angular distribution of the emitted infrared light, which is important for optical alignment and understanding the effective sensing area.

5. Mechanical & Package Information

5.1 Package Dimensions

The device uses a standard 5mm radial leaded package (often referred to as T-1¾). Key dimensions include the body diameter, lead spacing, and overall length. The drawing specifies that lead spacing is measured where leads emerge from the package. A maximum protrusion of resin under the flange is noted as 1.5mm. All dimensions have a standard tolerance of ±0.25mm unless otherwise stated.

5.2 Polarity Identification

For a standard IR emitter in this package, the longer lead is typically the anode (positive), and the shorter lead is the cathode (negative). The flat side on the package rim may also indicate the cathode side. Designers must verify this during assembly to prevent reverse connection.

6. Soldering & Assembly Guidelines

The datasheet provides specific instructions for soldering to prevent thermal damage to the semiconductor junction and the plastic package.

6.1 Hand or Wave Soldering

The absolute maximum rating specifies that leads can be soldered at 260°C for a maximum of 5 seconds, with the condition that the soldering point is at least 1.6mm (.063\") away from the package body. This distance allows heat to dissipate along the lead before reaching the sensitive components inside the package. Using a heat sink clip on the lead between the solder joint and the body is a recommended practice.

6.2 Storage Conditions

While not explicitly detailed beyond the storage temperature range (-55°C to +100°C), it is standard practice to store moisture-sensitive devices in a dry environment or in sealed, moisture-barrier bags with desiccant to prevent \"popcorning\" during reflow soldering, though this component is primarily for through-hole assembly.

7. Application Recommendations

7.1 Typical Application Scenarios

• Smoke Detectors: The UL approval makes it a primary choice. It is used in photoelectric smoke detectors where smoke particles scatter the IR beam from the emitter to a photodetector.
• Object/Slot Sensing: Paired with a matched phototransistor (e.g., LTR-3208) across a gap to detect the presence or absence of an object (paper in a printer, coin in a vending machine).
• Proximity Sensing: Used in systems where reflected IR light is detected to gauge distance or presence.
• Industrial Automation: For counting, positioning, and break-beam safety curtains.

7.2 Design Considerations

• Current Limiting: Always use a series resistor to limit the forward current to the desired value (e.g., 20 mA for spec measurements). Calculate resistor value using R = (V_supply - V_F) / I_F.
• Thermal Management: Account for the decrease in output with temperature (see Fig. 4). For high-temperature or high-current operation, ensure the power dissipation (I_F * V_F) does not exceed 250 mW and consider derating.
• Optical Alignment: The narrow 16-degree beam requires precise mechanical alignment with the detector for optimal signal strength.
• Electrical Noise: For pulsed operation, ensure fast drive circuitry and consider shielding to prevent electromagnetic interference from affecting sensitive detector circuits.

8. Technical Comparison & Differentiation

While a direct competitor comparison is not in the datasheet, key differentiators of the LTE-2872U can be inferred. Its primary advantage is the guaranteed matching to the LTR-3208 phototransistor series, reducing design uncertainty. The availability of multiple output bins allows for cost-performance optimization. The narrow viewing angle is a specific feature not found in all IR emitters; wider-angle emitters provide less intensity at a specific point but cover a larger area. The UL certification for smoke detectors is a significant qualification that not all IR LEDs possess, opening the door to a regulated market.

9. Frequently Asked Questions (Based on Technical Parameters)

Q1: What is the purpose of the different bins (A, B, C, D1, etc.)?
A1: The bins categorize the LEDs based on their measured radiant output (intensity). This allows you to select a component that meets the minimum output required for your application reliably. Using a higher bin ensures a stronger signal but may cost slightly more.

Q2: Can I drive this LED with a 5V supply directly?
A2: No. The typical forward voltage is 1.2-1.6V. Connecting it directly to 5V would cause excessive current, destroying the LED. You must always use a current-limiting resistor in series.

Q3: Why does the output drop at higher temperatures?
A3: This is a fundamental characteristic of semiconductor light sources. Increased temperature increases non-radiative recombination within the semiconductor material, reducing the efficiency of light generation (electroluminescence).

Q4: What does \"spectrally matched\" mean?
A4: It means the peak emission wavelength of the emitter (940nm) aligns closely with the peak spectral sensitivity wavelength of the specified phototransistor detector. This maximizes the amount of emitted light that the detector can \"see\" and convert into an electrical signal.

10. Practical Design Case Study

Scenario: Designing a Paper-Out Sensor for a Printer. A common application is to detect when paper is absent in a tray. An LTE-2872U IR emitter is placed on one side of the paper path, and an LTR-3208 phototransistor is placed directly opposite. When paper is present, it blocks the IR beam, and the phototransistor output is low (or high, depending on circuit configuration). When paper is absent, the beam reaches the detector, changing its output state. Design Steps: 1) Choose an appropriate bin (e.g., Bin C) for sufficient signal margin. 2) Design the driver circuit: Use a microcontroller GPIO pin. With a 3.3V supply and a target I_F of 20 mA, calculate R = (3.3V - 1.4V) / 0.02A = 95Ω. Use a standard 100Ω resistor. 3) Design the detector circuit: Connect the phototransistor in a common-emitter configuration with a pull-up resistor to create a digital signal. 4) Mechanically design the holder to ensure precise alignment of the emitter and detector across the paper path, utilizing the narrow 16-degree beam for accurate edge detection.

11. Principle of Operation Introduction

The LTE-2872U is a light-emitting diode (LED) that operates in the infrared spectrum. Its core principle is electroluminescence in a semiconductor p-n junction. When a forward voltage is applied, electrons from the n-type region and holes from the p-type region are injected into the junction region. When these charge carriers recombine, they release energy. In this specific material system (GaAlAs/GaAs), the energy released corresponds to a photon with a wavelength of approximately 940 nm, which is in the near-infrared region. The narrow beam is achieved through the geometry of the semiconductor die and the lensing effect of the clear plastic dome package, which collimates the emitted light.

12. Technology Trends and Context

Infrared emitters like the LTE-2872U are based on mature III-V semiconductor technology. Trends in the field include the development of emitters at different wavelengths (e.g., 850nm for some surveillance cameras, 1050nm for eye-safe applications) and with higher output powers and efficiencies. There is also a move towards surface-mount device (SMD) packages for automated assembly, though through-hole packages like this 5mm type remain popular for prototyping, repair, and applications requiring higher power handling or simpler manual assembly. The principle of matched emitter-detector pairs remains fundamental to reliable optoelectronic sensing. The integration of the emitter, driver, and sometimes the detector into a single module is another trend, simplifying system design for end-users.